A magnetic disk drive includes circular data tracks on a rotating magnetic disk and read and write heads that may form a merged head attached to a slider on a positioning arm. During a read or write operation, the merged head is suspended over the magnetic disk on an air bearing surface (ABS). The sensor in a read head is a critical component since it is used to detect magnetic field signals by a resistance change. One form of magnetoresistance is a spin valve magnetoresistance (SVMR) or giant magnetoresistance (GMR) which is based on a configuration in which two ferromagnetic layers are separated by a non-magnetic conductive layer in the sensor stack. One of the ferromagnetic layers is a pinned layer in which the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer in which the magnetization vector can rotate in response to external magnetic fields. The rotation of magnetization in the free layer relative to the fixed layer magnetization generates a resistance change that is detected as a voltage change when a sense current is passed through the structure. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the stack. Alternatively, there is a current-in-plane (CIP) configuration where the sense current passes through the sensor in a direction parallel to the planes of the layers in the sensor stack.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head in which the sensor is typically smaller than 0.1×0.1 microns at the ABS plane. Current recording head applications are still mainly based on an abutting junction configuration in which a hard bias layer is formed adjacent to each side of a GMR spin valve structure. As the recording density further increases and track width decreases, the junction edge stability becomes more important. A problem occurs when an edge demagnification effect leads to hard bias magnetization canting in which the in-plane magnetization vector in the hard bias layer is partially shunted away from the free layer. It is desirable to minimize this effect to achieve a better biasing direction. To achieve this objective, a higher hard bias coercivity is beneficial. Coercivity is defined as a measure of an opposing magnetic intensity which is required to remove a residual magnetism in a material that has been magnetized to saturation.
Referring to FIG. 1, a conventional read head 1 based on a GMR configuration is shown and is comprised of a substrate 2 upon which a first shield 3 and a first gap layer 4 are formed. There is a GMR element comprised of a bottom portion 5a, a free layer 6, and a top portion 5b formed on the first gap layer 4. Note that the GMR element generally has sloped sidewalls wherein the top portion 5b is not as wide as the bottom portion 5a. In addition, the GMR element may be a bottom spin valve in which an AFM pinning layer and pinned layer (not shown) are below the free layer 6 or the GMR element may be a top spin valve where the AFM and pinned layers are above the free layer. There is a seed layer 7 formed on the first gap layer 4 and along the GMR element which ensures that the subsequently deposited hard bias layers 8 have a proper microstructure. Leads 9 are provided on the hard bias layers 8 to carry current to and from the GMR element. The distance between the leads 9 defines the track width TW of the read head 1. Above the leads 9 and GMR element are successively formed a second gap layer 10 and a second shield 11.
The pinned layer in the GMR element is pinned in the Y direction by exchange coupling with an adjacent AFM layer that is magnetized in the Y direction by an annealing process. The hard bias layers 8 which are comprised of a material such as CoCrPt are magnetized in the X direction as depicted by vectors 15 and influence an X directional alignment of the magnetic vector 14 in the free layer 6. When a magnetic field of sufficient strength is applied in the Y direction from a recording medium by moving the head 1 over a hard disk (not shown) in the Z direction, then the magnetization in the free layer switches to the Y direction. This change in magnetic state is sensed by a voltage change due to a drop in the electrical resistance for an electrical current that is passed through the MR element. Unfortunately, as track width becomes smaller to enable increased recording density, the magnetization in the hard bias layers 8 tends to be shunted along the interface with the seed layer 7 near the junction with the GMR element. The loss of magnetization in the X direction represented by the vectors 12 causes a weakening in the magnetic coupling between the hard bias layers 8 and the free layer 6. Thus, the hard bias layers 8 may not provide sufficient longitudinal bias to stabilize the free layer 6 to achieve a single magnetic domain and undesirable Barkhausen noise may result. In order to overcome this condition, a hard bias layer with higher coercivity is needed.
Another concern with the conventional read head 1 is that the seed layer 7 may be non-magnetic and thereby weakens the magnetic coupling between the hard bias layers 8 and the free layer 6. Even when the seed layer 7 is magnetic, its coercivity is usually less than the hard bias layer which degrades device performance. Thus, a hard bias layer that does not require a seed layer and can be formed adjacent to a free layer in a magnetic read head is desirable.
The in-plane remnant magnetization (Mr) of the hard bias layers 8 represented by the vectors 13 in FIG. 1 must be higher than the Mr of the free layer 6 or the hard bias layers will not be able to stabilize the free layer and guarantee a single magnetic domain. The saturation magnetization (Ms) of the hard bias layers 8 is in the X direction. It follows that Ms and the squareness S=Mr/Ms of the hysteresis loop of the hard bias layer 8 along the in-plane (X direction) must be high for optimized performance of the read head 1.
Another requirement for the material in the hard bias layers 8 is that formation of an ordered microstructure is accomplished within a temperature range that does not affect the stability of the GMR element. Preferably, the deposition of the hard bias layer and subsequent annealing are performed at temperatures about 300° C. or less. This requirement precludes the implementation of ordered L10 type FePt or CoPt based materials that have a higher intrinsic magnetocrystalline anisotropy than currently used CoCrPt hard bias layers which have an Hc of about 2000 Oe as described by D. Weller in “Extremely High Density Longitudinal Magnetic Recording Media” found in Ann. Rev. Material Sci., Vol. 30, p. 611 (2000). It is noted that a standard FePt ordered structure requires a high temperature deposition (>400° C.) or high temperature anneal (>500° C.) that is not compatible with current recording head fabrication schemes. T. Maeda et al. in “Reduction of ordering temperature of an FePt ordered alloy by addition of Cu” in Applied Phy. Lett., Vol. 80, No. 12, p. 2147 (2002) report that the ordering temperature of FePt may be lowered to near 300° C. by adding a Cu dopant.
In U.S. Patent Application 2002/015268, a TiW seed layer is used to increase the Hc of an overlying FePt hard bias layer. Other body centered cubic (bcc) seed layers may be employed including a composite Cr/TiW seed layer. A FePt alloy is also used as a hard bias layer in U.S. Patent Application 2003/0193758. However, no conditions are provided in either publication for depositing or annealing the hard bias layer.
In U.S. Pat. No. 6,668,443, a NiCr smoothing layer is deposited on a substrate adjacent to a bottom spin valve. A seed layer may be formed on the NiCr layer. The smooth NiCr layer removes structural distortion in a subsequently deposited hard bias layer that results in improved longitudinal bias performance.
A Cr bias underlayer having a bcc lattice structure is disclosed in U.S. Pat. No. 6,608,740 and is formed between a CoCrPt hard bias layer and an MR element. This configuration provides the hard bias layer with a larger coercive force and a higher remanence ratio.
In U.S. Pat. No. 6,122,151, a hard bias layer such as CoCrPt is formed on a non-magnetic layer that is on a pinned layer in an MR element. The hard bias layer contacts the sides of the free layer. The non-magnetic layer is a metal layer such as Cr, Ti, Mo, or W. Optionally, the metal layer is a copper spacer layer in the MR element. An increase in coercivity, squareness ratio S, and bias magnetic field is observed. A method of reducing MR head instability is described in U.S. Pat. No. 6,462,920 and involves depositing a seed layer on a gap layer but not along the side of an MR element. A hard bias layer is formed on the seed layer and contacts the sides of the MR element to improve magnetic coupling therebetween.